Optical waveguide structure

ABSTRACT

An optical waveguide structure according to the invention comprises a core layer having a first refractive index n core , an array of sub-regions within the core having a second refractive index n rods , the array of sub-regions giving rise to a photonic band structure within the core layer, and a cladding layer adjacent to the core layer having a refractive index n cladding , wherein:  
     n core &gt;n rods ≧n cladding ,  
     The structure of the present invention is less lossy than prior waveguide structures having photonic bandstructure regions. The out of plane divergence of light in the sub-regions is reduced as compared with air holes which are typically used in photonic crystal structures. As a result more light is coupled back into the core at the sub-region/core interface. Coupling of light into the buffer layer is also reduced. Furthermore, there are added advantages over the prior art associated with the fabrication of these structures.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of optical waveguides and optical devices incorporating optical waveguides.

BACKGROUND TO THE INVENTION

[0002] It is increasingly recognised that integrated optical circuits have a number of advantages over electrical circuits. However, it has been difficult to produce integrated optical circuits which are comparably small, primarily due to the difficulty in producing waveguides which can include tight bends without large signal losses. It has also been difficult to produce integrated optical circuits including signal processing devices based on photonic band structures which can be easily coupled to current optical fibres, owing to a difference in the refractive index of the material used for optical fibres and those materials typically used for integrated optical devices, whilst still maintaining compact sizes.

[0003] Photonic crystals comprising a lattice of air holes formed in a core material, typically silicon or silicon nitride, have been fabricated, which exhibit a photonic band structure and typically a bandgap. Alternatively, a lattice of dielectric rods in air can be used. By not including some holes or rods in the lattice a line defect waveguide can be formed. Confinement of light within the waveguide is provided by using light within the photonic bandgap wavelength range. However, it has been found that devices of this type suffer from large losses, mainly due to the escape of light from the waveguide in a vertical direction.

[0004] Similarly, optical devices using this type structure for signal processing, such as filtering, suffer from large losses. This limits their usefulness.

SUMMARY OF THE INVENTION

[0005] According to a first aspect of the present invention, an optical waveguide structure comprises a core layer having a first refractive index n_(core), an array of sub-regions within the core having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure, and a cladding layer adjacent to the core layer having a refractive index n_(cladding), wherein:

n_(core)>n_(rods)≧n_(cladding).

[0006] Preferably, the array of sub-regions gives rise to a photonic bandgap.

[0007] The optical waveguide structure may be a planar structure. In this case, the waveguide guide structure preferably further includes a buffer layer having a refractive index n_(buffer), wherein the core layer is positioned between the buffer layer and the cladding layer and wherein:

n_(core)>n_(rods)≧n_(buffer).

[0008] Alternatively, the waveguide structure may be an optical fibre structure, wherein the cladding layer surrounds the core layer.

[0009] The present invention provides advantages over conventional photonic crystal devices which include an array of rods in air or an array of air holes formed in a core layer. In these conventional structures there is a large amount of loss for optical signals passing through them, especially out of the plane of propagation. The structure of the present invention is less lossy than prior waveguide structures having photonic bandstructure regions. The out of plane divergence of light in the sub-regions is reduced as compared with air holes which are typically used in photonic crystal structures. As a result more light is coupled back into the core at the sub-region/core interface. In the planar case, coupling of light into the buffer layer is also reduced. Furthermore, there are added advantages over the prior art associated with the fabrication of these structures.

[0010] The refractive index contrast between the core and the sub-regions affects the nature of the band structure. For some applications, such as filtering and dispersion compensation the difference in refractive index can be extremely small i.e. a difference in the third decimal place of the refractive index. However, other applications such as 90° bends in waveguides require a bandgap which overlaps in different propagation directions. This requires a much larger refractive index contrast. Preferably, the core layer has a refractive index between 1.4 and 4. Preferably, the sub-regions have a refractive index between 1.3 and 1.6. Preferably, the cladding has a refractive index between 1.3 and 1.6. In the planar case, preferably the buffer layer has a refractive index between 1.3 and 1.6.

[0011] Preferably, the sub-regions are formed from silicon oxynitride. Preferably, the core layer is formed from silicon nitride, doped silica, tantalum pentoxide or doped tantalum pentoxide. The cladding layer is preferably formed from silicon dioxide. In the planar case the buffer layer is preferably also formed from silicon dioxide.

[0012] The sub-regions may extend through the cladding layer as well as the core layer and partially or fully into the buffer layer. Alternatively, the cladding layer may include sub-regions corresponding to the sub-regions in the core layer having a refractive index which is greater than or equal to the refractive index of the cladding layer but which is less than or equal to the refractive index of the sub-regions in the core. Furthermore, in the planar case, the buffer layer may include sub-regions corresponding to the sub-regions in the core layer having a refractive index which is greater than or equal to the refractive index of the buffer layer but which is less than or equal to the refractive index of the sub-regions in the core.

[0013] The present invention is applicable to waveguides connecting integrated optical circuits as well as to individual optical devices which are used in integrated optical circuits. Any device incorporating waveguide bends in a glassy core layer can be improved by use of the present invention. Such devices include Arrayed Waveguide Gratings (AWGs), Mach Zehnder interferometers, directional couplers, dispersion compensators, splitters/multiplexers, polarisation compensators, optical switches, optical delay elements and filters.

[0014] Preferably, the core layer includes a lateral waveguiding region having no sub-regions. Preferably, the waveguiding region includes a waveguide bend.

[0015] According to a second aspect of the invention, a method of manufacturing a optical waveguide structure comprises the steps of:

[0016] providing a core layer having a first refractive index n_(core);

[0017] providing an array of sub-regions within the core having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure; and

[0018] providing a cladding layer adjacent to the core layer having a refractive index n_(cladding); wherein:

n_(core)>n_(rods)≧n_(cladding).

[0019] The optical waveguide may be planar, the method further including the step of providing a buffer layer having a refractive index n_(buffer) on the opposite side of the core layer to the cladding layer, wherein:

n_(core)>n_(rods)≧n_(buffer).

[0020] Alternatively, the optical waveguide may be an optical fibre, the method further including the steps of:

[0021] providing the cladding layer surrounding the core layer.

[0022] According to a third aspect of the present invention, a method of guiding an optical signal comprises the step of passing an optical signal through a waveguiding region of an optical waveguide structure comprising a core layer having a first refractive index n_(core), an array of sub-regions within the core layer having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure, and a cladding layer adjacent the core layer having a refractive index n_(cladding), wherein:

n_(core)>n_(rods)≧n_(cladding).

[0023] The optical waveguide structure may be a planar structure. In this case, the waveguide guide structure preferably further includes a buffer layer having a refractive index n_(buffer), wherein the core layer is positioned between the buffer layer and the cladding layer and wherein:

n_(core)>n_(rods)≧n_(buffer).

[0024] Alternatively, the waveguide structure may be an optical fibre structure, wherein the cladding layer surrounds the core layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

[0026]FIG. 1 is a schematic cross sectional view of a photonic crystal embedded in a waveguide structure in accordance with the prior art;

[0027]FIG. 2a is a schematic cross sectional view of a photonic crystal embedded in a waveguide structure in accordance with the present invention;

[0028]FIGS. 2b and 2 c are schematic cross sectional views of other examples of photonic crystals embedded in a waveguide structure in accordance with the present invention;

[0029]FIG. 3 shows a waveguide design in accordance with the present invention;

[0030]FIG. 4 shows a waveguide bend formed with a waveguide design in accordance with the present invention;

[0031]FIG. 5 is a schematic illustration of a optical device in accordance with the present invention; and,

[0032]FIG. 6 shows an optical fibre incorporating a structure in accordance with present invention.

DETAILED DESCRIPTION

[0033] Photonic crystal waveguide structures are based on some perturbation in dielectric constant in the core of a planar waveguide structure. This has most commonly been performed by the etching of air rods into the core layer of the waveguide. As light propagates through the core it interacts with the dielectric constant modulation and, in some structures, in a manner analogous to electrons in a semiconductor, certain electromagnetic fields are forbidden to propagate in the core. The forbidden electromagnetic fields form a photonic bandgap. More detail on the nature of the band structure of photonic crystals of this sort can be found in WO98/53351, (BTG International Limited).

[0034]FIG. 1 illustrates the interaction of the electric field (E-field) of an optical mode with the core 1 in a photonic crystal according to the prior art. The light is travelling through the core 1 from left to right. A profile of the E-field within the core 1, cladding 2 and buffer 3 layers is shown. It can be seen that in the photonic crystal region the mode confinement is reduced and there is out of plane loss. When the light reaches the first air/core interface, the light diverges strongly in the vertical direction, introducing loss. Once the light is in the air region 4 there is no confinement and light escapes from the top of the structure and into the buffer layer 3, which is of a higher refractive index than air. Furthermore, owing to the fact that the structure is not symmetric, and light is not well confined in the vertical direction, light leaks into the buffer layer 3 from the air rods 4.

[0035] Vertical loss in the waveguide structure is very significant and limits the usefulness of the structure in practical devices, especially in confinement applications such as in waveguide bends.

[0036]FIG. 2a shows a waveguide structure according to one aspect of the present invention. The waveguide structure shown in FIG. 2 comprises a core layer 10, having a refractive index n_(core), an array of rods 11 in the core layer 10 having a refractive index n_(rods), and buffer 12 and cladding layers 13 having a refractive index n_(buffer) and n_(cladding), respectively. In this example the rods 11 extend through the cladding layer 13 and partially into the buffer layer 12. However, alternatively, the rods may be formed solely in the core layer or solely in the core layer and cladding layer. The refractive indices satisfy the inequality:

n_(core)>n_(rods)≧n_(cladding) and n_(buffer)

[0037] This condition provides greater vertical confinement of the E-field of an optical signal passing through the waveguide. The higher refractive index of the rods 11 reduces the tendency of the light to leak into the buffer layer 12 and reduces losses from the top of the structure and into the substrate.

[0038]FIG. 2b shows an another example of a waveguide structure. The structure is identical to the structure shown in FIG. 2a in that it has substrate 14, buffer 12, core 10 and cladding 13 layers. The only difference is that the rods 15 extend through the cladding 13 and the core 10, but not into the buffer 12. Similarly, FIG. 2c shows a waveguide structure with substrate 14, buffer 12, core 10 and cladding 13 layers but in this example the rods 16 exist only in the core layer 10.

[0039] The core 10 material of the structure of FIG. 2a is around a micron in thickness and may be formed of silicon nitride (n=2.02). The rods 11 may be composed of silicon oxynitride (n=1.6). The cladding 13 and buffer 12 layers are formed of silicon dioxide (n=1.46). The buffer 12 and cladding 13 layers need not be formed of the same material as long as they satisfy the inequality above. The materials described above are examples only. The benefit of the invention will be realised as long as the inequalities are satisfied. However, for structures which are easily coupled to typical optical fibres and devices it is preferred that the core layer has a refractive index between 1.4 and 4 and more preferably between 1.4 and 2.5, the rods have a refractive index between 1.3 and 1.6 and the cladding and buffer layers each have a refractive index between 1.3 and 1.6.

[0040] The waveguide of FIG. 2a also includes a substrate layer 14 underneath the buffer layer 12. The waveguide structure of FIG. 2a may be fabricated as follows. The buffer layer 12 is put on the substrate by thermal oxidation, HIPOX or plasma enhanced chemical vapour deposition (PECVD) depending on whether a thin or thick oxide is being deposited. The core layer is put down next by PECVD, CVD or sputtering. The position of the rods 11 is then defined, for example, by etching into the core 10. Wet or dry etching may be used but dry etching is preferred. The position of the rods may be either direct-written using an e-beam, or transferred from a mask. The material filling the rods, in this case silicon oxynitride, is then deposited into the etched holes using any suitable technique, such as PECVD, chemical vapour deposition (CVD), molecular beam epitaxy (MBE) or sputtering. Any silicon oxynitride on top of the waveguide can be removed preferably by dry etching, but alternatively by controlled wet etching or chemical mechanical polishing. The cladding layer is then deposited by PECVD, CVD or sputtering.

[0041] In the case described above both the filling material and the cladding are different materials. In order to simplify fabrication, the material filling the rods may be the same as the cladding. With a core of silicon nitride (n=2.02) and rods of silicon oxynitride (n=1.6), the silicon oxynitride (n=1.6) on top of the waveguide during fabrication can be retained. This provides a filling material which is identical to that of the cladding, which satisfies n_(core)×n_(rods)=n_(cladding). Alternatively, rods can be grown or etched from the substrate and a waveguide structure grown around the rods.

[0042] Additionally, it is possible to include a different material to define the rods in the buffer and cladding layers, with a refractive index n_(rods in cladding and buffer). In this instance the following inequality applies:

n_(core)>n_(rods in core)>n_(rods in cladding and buffer)>n_(cladding) and n_(buffer)

[0043] This type of structure improves transmittance but is more difficult to fabricate. The buffer layer 23 is deposited on a substrate 25, the rods are defined and etched partially into the buffer. A low index silicon oxynitride is deposited into the rods. The remaining silicon oxynitride is removed. The core layer 20 is deposited and the rods are defined and etched into the core. A slightly higher index silicon oxynitride is deposited into the rods 21 in the core 20 and the remaining silicon oxynitride is removed. The cladding layer 24 is then deposited and the rods are defined again. The rods are etched into the cladding and filled with a lower index silicon oxynitride. This results in the structure shown in FIG. 3. An example of refractive indices for this embodiment is n_(core=2.02), n_(rods in core)=1.6, n_(rods in cladding and buffer)=1.58 and n_(cladding) and n_(buffer)=1.46.

[0044] As shown in FIG. 4, waveguides in accordance with the present invention can include tight waveguide bends. The waveguide structure comprises an array of silicon oxynitride rods 30 extending through a cladding layer 31 and a core layer 32 and partially into a buffer layer 33, formed on a substrate 34. A number of rods are missing from the array forming a waveguide which includes a 90° bend. Clearly, the waveguide could take any shape and could, for example, include a bifurcation to form a splitter. The reduced vertical loss from the waveguide means that light within the bandgap of the photonic crystal region is confined with the waveguide and is forced to propagate around the bend. This allows integrated optical circuits to be fabricated over a much smaller area with greatly reduced loss (of the order of 10 dB) and optical devices incorporating waveguide bends to be made smaller.

[0045] Other devices may also be made incorporating a photonic band structure in an optical waveguide in accordance with the present invention, such as AWGs, multiplexers, demultiplexers and dispersion compensators. These devices are formed in the same manner as described in WO98/53351 (BTG International Limited) referenced above, but with materials chosen to satisfy n_(core)>n_(rods)≧n_(cladding). FIG. 5 is a schematic illustration of such an optical device 35, including an optical input 36 and an optical output 37. The device 35 typically includes a photonic band structure region in the optical path of an input optical signal which acts to process the signal in some way, such as dispersion compensating.

[0046] The present invention can be applied to any glass technology, whether it is planar or fibre. For example, as shown in FIG. 6, conventional fibre 40 could be flattened or planarised and an array of filled holes 41 incorporated into the flattened region through the cladding 42 and the core 43. The structure as a whole remains in-fibre.

[0047] The material forming the high index rods is not necessarily silicon oxynitride, it may for example be a non-linear material of suitable refractive index, providing the possibility of a tuneable device, for example a tuneable filter.

[0048] The present invention provides a waveguiding structure having a photonic band structure with lower loss than prior structures of the same type. This means that a larger number of rows of rods, equating to conventional holes, can be used in a device structure for the same amount of loss. High losses in prior structures has limited the effect of the band structure. With the present invention it is feasible to produce longer structures for the same loss, and hence longer time delays and higher resolution filters and demultiplexers. 

1. An optical waveguide structure comprising a core layer having a first refractive index n_(core), an array of sub-regions within the core having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure, and a cladding layer adjacent to the core layer having a refractive index n_(cladding), wherein: n_(core)>n_(rods)≧n_(cladding).
 2. An optical waveguide structure according to claim 1, wherein the array of sub-regions gives rise to a photonic bandgap.
 3. An optical waveguide structure according to claim 1, wherein the waveguide structure is a planar waveguide structure further including a buffer layer having a refractive index n_(buffer), wherein the core layer is positioned between the buffer layer and the cladding layer and wherein: n_(core)>n_(rods)≧n_(buffer).
 4. An optical waveguide structure according to claim 1, wherein the waveguide structure is an optical fibre structure, the cladding layer surrounding the core layer.
 5. An optical waveguide structure according to claim 1, wherein the core layer has a refractive index between 1.4 and
 4. 6. An optical waveguide structure according to claim 1, wherein the sub-regions have a refractive index between 1.3 and 1.6.
 7. An optical waveguide structure according to claim 1, wherein the cladding layer has a refractive index between 1.3 and 1.6.
 8. An optical waveguide structure according to claim 3, wherein the buffer layer has a refractive index between 1.3 and 1.6.
 9. An optical waveguide structure according to claim 1, wherein the sub-regions are formed from silicon oxynitride or silicon dioxide.
 10. An optical waveguide structure according to claim 1, wherein the core layer is formed from silicon nitride, doped silica, tantalum pentoxide or doped tantalum pentoxide.
 11. An optical waveguide structure according to claim 1, wherein the cladding layer is formed from silicon dioxide.
 12. An optical waveguide structure according to claim 3, wherein the buffer layer is formed from silicon dioxide.
 13. An optical waveguide structure according to claim 1, wherein the sub-regions extend through the cladding layer as well as the core layer.
 14. An optical waveguide structure according to claim 3, wherein the sub-regions extend partially or fully into the buffer layer.
 15. An optical waveguide structure according to claim 1, wherein the cladding layer includes sub-regions corresponding to the sub-regions in the core layer having a refractive index which is greater than or equal to the refractive index of the cladding layer but which is less than or equal to the refractive index of the sub-regions in the core.
 16. An optical waveguide structure according to claim 1, wherein the core layer includes a lateral waveguiding region having no sub-regions.
 17. An optical waveguide structure according to claim 16, wherein the waveguiding region includes a waveguide bend.
 18. An optical device including an optical waveguide structure according to claim
 1. 19. A method of manufacturing a optical waveguide structure comprising the steps of: providing a core layer having a first refractive index n_(core); providing an array of sub-regions within the core having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure; and providing a cladding layer adjacent to the core layer having a refractive index n_(cladding); wherein: n_(core)>n_(rods)≧n_(cladding).
 20. A method according to claim 19, wherein the optical waveguide is planar, the method further including the step of providing a buffer layer having a refractive index n_(buffer) on the opposite side of the core layer to the cladding layer, wherein: n_(core)>n_(rods)≧n_(buffer).
 21. A method according to claim 19, wherein the optical waveguide is an optical fibre, the method further including the steps of: providing the cladding layer surrounding the core layer.
 22. A method of guiding an optical signal comprises the step of passing an optical signal through a waveguiding region of an optical waveguide structure comprising a core layer having a first refractive index n_(core), an array of sub-regions within the core layer having a second refractive index n_(rods), the array of sub-regions giving rise to a photonic band structure experienced by an optical mode travelling through the waveguide structure, and a cladding layer adjacent the core layer having a refractive index n_(cladding), wherein: n_(core)>n_(rods)≧n_(cladding).
 23. A method according to claim 22, wherein the optical waveguide structure is a planar structure, further including a buffer layer having a refractive index n_(buffer), wherein the core layer is positioned between the buffer layer and the cladding layer and wherein: n_(core)>n_(rods)≧n_(buffer).
 24. A method according to claim 22, wherein the waveguide structure is an optical fibre structure, wherein the cladding layer surrounds the core layer. 